1. Field of the Invention
This invention relates to a photocurable composition which is developable with an aqueous alkaline solution and is applicable particularly advantageously to the formation of a conductor circuit pattern, a barrier rib pattern, a dielectric pattern, a fluorescent pattern, and a black matrix on the front and the back substrate of a plasma display panel (PDP), and applicable also to the formation of an electric conductor, a resister, and a dielectric element for use in a fluorescent display tube and electronic parts. This invention also relates to such calcined patterns as a conductor pattern, a vitreous dielectric pattern, and a fluorescent pattern which are obtained by the use of the photocurable composition.
2. Description of the Prior Art
In recent years, a front substrate and back substrate of a plasma display panel, an electrode circuit substrate of a printed circuit board, and the like have been witnessing steady advance of the patterns formed thereon toward higher fineness. In consequence of this trend, the desirability of improving the technique for the formation of such patterns has been finding growing approval. Particularly, the plasma display panels have been undergoing conspicuous technological innovations devoted to enlarging size and enhancing resolution. The manufacturers of plasma display panels have recently succeeded in commercializing such panels of the 50-inch class and have been continuing an effort to produce such panels in a larger size with higher resolution.
Heretofore, the conductor patterns and dielectric patterns in plasma display panels, fluorescent display tubes, electronic parts, etc. have been generally formed by the screen printing process which uses an electroconductive paste or a glass paste containing a very large amount of metal powder or glass powder. The formation of such patterns by the screen printing process, however, entails such problems as requiring skill on the part of workers, exposing the layer of fresh paint deposited by printing to the possibility of sustaining blurs or blots, degrading the accuracy of registration of printed patterns due to expansion and contraction of the screen, and jagging the formed pattern due to the contact with the screen mesh. Thus, the screen printing process produces required patterns only with a poor yield and incurs difficulty in coping with the trend of patterns toward higher fineness and the trend of plasma display panels toward enlargement of size. In the circumstances, the desirability of developing a pattern processing material which is capable of more stably coping with the demand for patterns of higher fineness and for panels of larger size has been finding recognition.
As a prospective alternative for the screen printing process, therefore, the photolithographic process has been proposed in published Japanese Patent Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as xe2x80x9cJP-A-xe2x80x9d) 1-296,534, JP-A-2-165,538, and JP-A-5-342,992, for example. The photolithographic process forms a pattern by applying an ultraviolet-curable glass paste to an insulating substrate and exposing and developing the coating of paste.
As to the development step in the photolithographic process, with due respect to the problem of environmental safety, the alkali development type has come to play the leading role. A macromolecular compound containing carboxyl groups is generally used as a film-forming component for the purpose of modifying a given composition to the alkali development type.
When the macromolecular compound containing carboxyl groups is made to incorporate such a fine basic inorganic powder as glass frit, however, the paste consequently obtained acquires too inferior viscosity stability to fit practical use. Specifically, the glass paste composition of such inferior viscosity stability entails deterioration of the operational efficiency of coating work and degradation of the developing properties of the applied coating in consequence of such phenomena as gelation and decline of flowability and consequently brings the problem of affording no sufficient allowance in work.
A primary object of the present invention, therefore, is to provide an alkali development type photocurable composition which is excellent in storage stability (viscosity stability), operational efficiency of coating work, and developability with an aqueous alkaline solution notwithstanding an extremely large content of a fine inorganic powder and, at the same time, capable of forming a pattern of high fineness and high aspect ratio without giving rise to curls (warps) or peeling along the pattern edges during the course of calcining.
A further object of the present invention is to provide an alkali development type photocurable composition which enjoys an excellent calcining properties, permits use of a relatively low temperature for the purpose of calcining, and exhibits stable fastness of adhesion to a substrate invariably at the steps of drying, exposure to light, development, and calcining.
A more concrete object of the present invention is to provide an alkali development type photocurable composition which permits formation of a conductor circuit pattern, a vitreous dielectric pattern, or a fluorescent pattern of high fineness with satisfactory workability and high productivity by the photolithographic technique, and undergoes a calcining step at a temperature of not more than 600xc2x0 C. infallibly without giving rise to a calcining residue capable of exerting an adverse effect on an image.
Another object of the present invention is to provide a calcined pattern of high fineness manufactured with high productivity from the photocurable composition mentioned above through a series of selective exposure to light, development, and calcining and a technique for the manufacture.
To accomplish the objects mentioned above, the first aspect of the present invention resides in providing an alkali development type photocurable composition, which composition is characterized by comprising (A) an alkali-soluble macromolecular binder having a weight-average molecular weight in the range of 5,000 to 100,000 and an acid value in the range of 50 to 150 mg KOH/g, possessing no ethylenically unsaturated double bond, and obtained by causing (d) a compound possessing one glycidyl group and no ethylenically unsaturated double bond in its molecule to react with a carboxyl group of (A-1) a copolymer of (a) an ethylenically unsaturated bond-containing compound possessing one carboxyl group in its molecule with (b) an ethylenically unsaturated bond-containing compound possessing neither hydroxyl group nor acidic group in its molecule, the copolymer possessing no glycidyl group, or with a carboxyl group of (A-2) a copolymer of (a) an ethylenically unsaturated bond-containing compound possessing one carboxyl group in its molecule with (b) an ethylenically unsaturated bond-containing compound possessing neither hydroxyl group nor acidic group in its molecule and (c) an ethylenically unsaturated bond-containing compound possessing a hydroxyl group, the copolymer possessing no glycidyl group, and then causing (e) a polybasic acid anhydride to react with a secondary hydroxyl group caused by the above reaction and a primary hydroxyl group of the copolymer (A-2) mentioned above, (B) an inorganic powder, (C) a photopolymerizable monomer, (D) a photopolymerization initiator, and (E) an organic solvent. Preferably, the inorganic powder (B) mentioned above contains a low melting glass frit account for a portion of not less than 5% by weight thereof.
The photocurable composition of the present invention may be in the form of paste or in the form of a dry film produced in advance from the composition in the form a film.
In the case of the pasty form, a photocurable and electroconductive past composition is produced by using a fine electroconductive powder as the inorganic powder (B) and a photocurable glass paste composition is produced by exclusively using glass powder as the inorganic powder (B). The paste composition for use in the formation of the black pattern additionally contains a black pigment.
The inorganic powder mentioned above is advantageously used when it has an average particle diameter of not more than 10 microns.
As the fine electroconductive powder which is effectively used for the photocurable and electroconductive paste, any powder of an electroconductive metal such as ruthenium, gold, silver, copper, palladium, platinum, aluminum, and nickel and other black fine electroconductive powders may be used. In the case of the photocurable glass paste, a low melting glass having a softening point in the range of 300xc2x0 to 600xc2x0 C. may be advantageously used.
As the black pigment, those mainly formed of a metal oxide containing as a main component thereof one or more metals selected from among Fe, Co, Cu, Cr, Mn, and Al may be advantageously used.
Since the contact of the carboxyl groups of an alkali-soluble macromolecular binder with fine basic inorganic particles are repressed owing to the steric hindrance between the main chain and the side chain of the binder, the alkali development type photocurable composition of the present invention, though developable with an aqueous alkaline solution, excels in viscosity stability (storage stability) and avoids bringing such problems as deteriorating the operational efficiency of coating work due to gelation or decline of flowability of the composition or degrading the pattern-forming properties due to decline of the developing properties. Further, when the photocurable composition contains such an alkali-soluble macromolecular binder in combination with a stabilizer, it exhibits excellent storage stability even if it contains a large amount of the inorganic powder.
Another aspect of the present invention resides in providing a calcined pattern formed of the photocurable composition described above. When the photocurable composition is in the form of paste, for example, the pasty photocurable composition is applied to a substrate and then dried to form a film. When the photocurable composition is in the form of a dry film, the dry film is laminated on the substrate. A calcined pattern of high fineness is obtained by patterning the superposed layer of the composition by selective exposure to light and development, and thereafter calcining the patterned film.
The calcined pattern which is formed as described above serves as an electroconductive pattern when a fine metallic powder is used as the inorganic powder (B) mentioned above or a vitreous dielectric pattern when a glass powder is used instead. Optionally, a fluorescent pattern may be formed by using a fluorescent powder as the inorganic powder.
The use of the photocurable composition of the present invention, therefore, enables such calcined patterns of high fineness as conductor pattern, vitreous dielectric pattern, and fluorescent pattern to be stably formed with high yield and satisfactory productivity on a substrate of a large surface area by the technique of photolithography without entailing such problems as deteriorating the working environment.